A senior UN auditor has launched a blistering attack on UN Secretary General Ban Ki-moon's leadership.

Inga Britt Ahlenius accused Mr Ban of undermining her efforts and of leading the UN into an era of decline.

The claims come in a leaked report which reflects the acrimony over power struggles surrounding the UN's internal investigations process.

Ms Ahlenius is the outgoing head of a body charged with combating corruption and misconduct within the organisation.

In the report, leaked to a US newspaper, Ms Ahlenius accuses Ban Ki-moon of attempting to undercut the independence of her "oversight" office.

She claims he tried to establish a parallel investigation apparatus under his own authority, and blocked her ability to recruit senior staff.

'Process of decay'

Mr Ban's chief of staff rejects the accusations.

He says the secretary general's actions were intended to strengthen the UN's ability to fight corruption. And Mr Ban had been obliged to address a recruitment exercise that did not comply with UN policies, the chief of staff added.

Whatever the case, Ms Ahlenius' memo is an extraordinary personal attack on the secretary general.

She claims that under Mr Ban's leadership the UN secretariat is in a process of decay, drifting in the absence of strategic guidance, and weakening the overall position of the United Nations.

Mr Ban's performance has had mixed reviews during his three and a half years in office.

Critics describe him as ineffectual and lacking in charisma, while supporters applaud him for taking the lead on issues including climate change and women's empowerment.